The present invention relates to removal of metal oxide scale which forms on metal products, and more particularly to a pickling method for stripping such scale from processed metal products, such as steel.
Rolled, forged and heat treated metal products are formed by hot processes into many different shapes, such as flat sheet and bar. Metal products may be further processed by annealing, pickling, and cold rolling. With respect to steel products, cold rolling produces a finished steel material that has a smoother finish and more accurate dimensions than a non-cold rolled product, and furthermore, hardens the steel material to provide a stronger product. These finished materials are then sold to fabricators for manufacture into a wide variety of products. For example, sheet steel is sold to automobile manufacturers for use in automobile bodies.
With respect to the manufacture of steel sheet, in large integrated steel mills, steel slabs are rolled into sheet of about 0.05-0.25 inch thickness and then rolled into coils weighing up to about 20-40 tons. The slabs are rolled in the red-hot condition. The coiled sheet is referred to as a hot band. After coiling, the hot band is allowed to cool before it is processed for use in finished product. Steel bars and other shapes may also be manufactured using any of various hot processes and then allowed to cool before subsequent processing. In any case, during the manufacturing process, layers of material collectively referred to as mill scale may form over the surface of the steel.
One particular aspect of the mill scale is a layer of metal oxide scale that typically forms as the hot band cools. The metal oxide scale that forms on the steel results from a chemical oxidation reaction and typically comprises three phases of iron oxide, namely, FeO, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4. The thickness of that iron oxide scale and the relative amounts of the iron oxide phases will vary with temperature, grade of steel and cooling rates, for example.
The iron oxide scale layer present on the steel is known to interfere with subsequent processing and use of the steel. With sheet steel, for example, the scale layer may abrade adjacent portions of steel in the coil thereby ruining the steel surface for subsequent use. Similarly, the scale can adversely affect the equipment used to process the steel. It may even be impossible to use steel for further processes such as cold forming if the product quality is poor due to the metal oxide scale layer.
Thus, prior to subsequent processing, it has been known to expose the steel to a pickling process in which the metal oxide layer is chemically removed from the metal surface by action of water solutions of inorganic acids. In one such process used for sheet steel, the hot band is uncoiled, and the sheet steel passed through a series of acid tanks and rinsing tanks in a continuous or semi-continuous pickling line. As the sheet passes through the acid tanks, the acid solution, which is typically subjected to agitation, removes the oxide scale from the surface of the sheet steel. For bar stock and other shapes, bundles referred to as lifts are pickled in a batch process, i.e., the lift is immersed in an acid tank and held therein while the acid solution is agitated such as by stirring, until the scale layer is removed. In some cases, it may be necessary to mechanically disrupt the scale layer in order for the acid solution to effectively remove the scale. For this purpose, scale breakers may be employed. Prior to pickling sheet steel, for example, the sheet is passed through a pair of rollers which reduce the thickness of the scale layer and open up the scale surface for attack by the acid in the tank.
When stainless steel is hot processed, a metal oxide scale forms on the surface, similar to the iron oxide formation on low carbon steel. To make the final stainless steel product shiny in appearance, flat, and to prevent damage to subsequent processing equipment, the stainless steel products also are pickled using the same or a similar acid pickling process to those described above. Additionally, products that are to be galvanized or electro-galvanized, a process whereby a zinc coating is deposited onto the steel to form a protective layer, also are first pickled to remove surface oxide layers to expose the steel for the zinc coating.
In addition to steel products, other metal products develop metal oxide scale as a result of hot processing. By way of example and not limitation, aluminum, zirconium, zinc, copper, alloys thereof and other metals and alloys also form oxide scales during manufacturing that must be removed by a pickling process.
Such pickling processes, while widely used to remove metal oxide scale, have many limitations and drawbacks. One particular concern with the pickling process is the throughput rate, i.e., the amount of metal that can be pickled in a set amount of time. The throughput rate is limited in that the steel or other metal must be exposed to the acid solution long enough to fully clean the metal surface of the scale layer. The time necessary to accomplish full pickling is affected by numerous variables, including solution temperature and concentration, agitation and time of immersion.
In an effort to minimize the time necessary to effect complete scale removal, the steel industry routinely employs hot sulfuric or hot hydrochloric acid for the pickling process. These acids are highly caustic, and are heated to temperatures at or above 140.degree. F. (60.degree. C.) for batch pickling processes, and between about 200.degree. F. and 220.degree. F. (93.degree. C. and 104.degree. C.) for the continuous and semi-continuous pickling processes. Such caustic acids, especially at high temperatures, present significant environmental and safety hazards.
Some attempts that have been made to improve the pickling process have centered on methods of arranging nozzles, spray pressures and segmentation of acid into zones to reduce drag-out from one tank to the next. These methods, however, continue to rely on high acid concentrations and temperatures.
Sumita et al. U.S. Pat. No. 4,588,488 proposes to reduce the temperatures and concentrations used in the pickling process for steel with an electron injection method based on cathodic polarization. In this method, a platinum electrode and steel part are immersed in a wash liquid containing an electrolyte, a pH regulating agent and a complexing agent. The positive cathode of an external dc power source is coupled to the electrode, and the negative anode is coupled to the steel. By imposing a positive voltage across the electrode to the steel, the oxide layer is said to be brought into an unstable region by shifting the potentials of the oxides in the base direction from the natural potential to the cathodic polarization potential. At this potential, the oxide is said to be unstable and will dissolve while the metal iron is stable and protected from corrosion. By this method, it is said to be possible to reduce the temperature and acid concentration of the wash bath while still achieving acceptable stripping times.
The method proposed in the Sumita et al. patent is believed to have many deficiencies, and is not readily applicable in the context of a steel mill, for example. On the one hand, the potential that is applied must be regulated and adjusted in accordance with the actual potentials encountered during the process. The nature of the oils that build up in a pickling tank, and the behavior of the materials therein, make it difficult, if not practically impossible, to monitor the various components and properly control the applied potential. Moreover, the electron injection method is not believed to produce any meaningful improvement in pickling process throughput.
There thus remains a need in the metal manufacturing industries, such as in the steel industry, to improve pickling processes for metal oxide scale removal to not only allow for a reduction in acid concentrations and temperatures, but to do so in a simple and realizable manner that does not adversely affect pickling times, but instead may actually speed up the pickling process.